The diagram below illustrates the relationship between QRS axis and the frontal leads of the ECG.

**Normal Axis**= QRS axis between -30 and +90 degrees.**Left Axis Deviation**= QRS axis less than -30 degrees.**Right Axis Deviation**= QRS axis greater than +90 degrees.**Extreme Axis Deviation**= QRS axis between -90 and 180 degrees (AKA “Northwest Axis”).

There are several complementary approaches to estimating QRS axis, which are summarised below.

#### Method 1 – The Quadrant Method

The most efficient way to estimate axis is to look at leads I + aVF.

Lead I | Lead aVF | Quadrant | Axis |
---|---|---|---|

Positive | Positive | Left lower quadrant | Normal (0 to +90 degrees) |

Positive | Negative | Left upper quadrant | Possible LAD (0 to -90 degrees) |

Negative | Positive | Right lower quadrant | RAD (+90 to 180 degrees) |

Negative | Negative | Right upper quadrant | Extreme Axis Deviation (-90 to 180 degrees) |

#### Method 2 – Leads I + II

Another rapid method is to look at leads I + II.

A positive QRS in lead I puts the axis in roughly the same direction as lead I.

A positive QRS in lead II similarly aligns the axis with lead II.

Therefore, if leads I and II are *both* positive, the axis is between -30 and +90 degrees (i.e. normal axis).

#### Combining Methods 1 and 2

By combining these two methods, you can rapidly and accurately assess axis.

Lead I | Lead aVF | Axis |
---|---|---|

Positive | Positive | Normal (0 to +90 degrees) |

Positive | Negative | Possible LAD Is lead II positive? Yes -> Normal (0 to -30 degrees) No -> LAD (-30 to -90 degrees) |

Negative | Positive | RAD (+90 to 180 degrees) |

Negative | Negative | Extreme Axis Deviation (-90 to 180 degrees) |

#### Method 3 – The Isoelectric Lead

This method allows a more precise estimation of QRS axis, using the axis diagram below.

#### Key Principles

- If the QRS is positive in any given lead, the axis points in roughly the same direction as this lead.
- If the QRS is negative in any given lead, the axis points in roughly the opposite direction to this lead.
- If the QRS is
*isoelectric*in any given lead (positive deflection = negative deflection), the axis is at 90 degrees to this lead.

**Step 1. Find the isoelectric lead.**

The isoelectric (equiphasic) lead is the frontal lead with zero net amplitude. This can be either:

- A biphasic QRS where R wave height = Q or S wave depth.
- A flat-line QRS with no discernible features.

**Step 2. Find the positive leads. **

- Look for the leads with the tallest R waves (or largest R/S ratios).

**Step 3. Calculate the QRS axis. **

- The QRS axis is at 90 degrees to the isoelectric lead, pointing in the direction of the positive leads.

*This concept can be difficult to understand at first, and is best illustrated by some examples.*

#### Example 1

- Leads I + aVF are both positive.
- This puts the axis in the left lower quadrant, between 0 and +90 degrees, i.e. normal axis.
- Lead II is also positive, which confirms the normal axis.

- Lead aVL is isoelectric, being biphasic with similarly sized positive and negative deflections (no need to precisely measure this).
- From the diagram above, we can see that aVL is located at -30 degrees.
- The QRS axis must be ± 90 degrees from lead aVL, either at +60 or -120 degrees.
- With leads I (0), II (+60) and aVF (+90) all being positive, we know that the axis must lie somewhere between 0 and +90 degrees.
- This puts the QRS axis at +60 degrees.

#### Example 2

- Lead I = negative.
- Lead aVF = positive.
- This puts the axis in the right lower quadrant, between +90 and +180 degrees, i.e. RAD.

- Lead II (+60 degrees) is the isoelectric lead.
- The QRS axis must be ± 90 degrees from lead II, at either +150 or -30 degrees.
- The more rightward-facing leads III (+120) and aVF (+90) are positive, while aVL (-30) is negative.
- This puts the QRS axis at +150 degrees.

*This is an example of right axis deviation secondary to right ventricular hypertrophy.*

#### Example 3

- Lead I = positive.
- Lead aVF = negative.
- This puts the axis in the left upper quadrant, between 0 and -90 degrees, i.e. normal or LAD.
- Lead II is neither positive nor negative (isoelectric), indicating borderline LAD.

- Lead II (+60 degrees) is isoelectric.
- The QRS axis must be ± 90 degrees from lead II, at either +150 or -30 degrees.
- The more leftward-facing leads I (0) and aVL (-30) are positive, while lead III (+120) is negative.
- This confirms that the axis is at -30 degrees.

*This is an example of borderline left axis deviation due to inferior MI.*

#### Example 4

- Lead I = negative.
- Lead aVF = negative.
- This puts the axis in the upper right quadrant, between -90 and 180 degrees, i.e. extreme axis deviation.

*NB. The presence of a positive QRS in aVR with negative QRS in multiple leads is another clue to the presence of extreme axis deviation. *

- The most isoelectric lead is aVL (-30 degrees).
- The QRS axis must be at ± 90 degrees from aVL at either +60 or -120 degrees.
- Lead aVR (-150) is positive, with lead II (+60) negative.
- This puts the axis at -120 degrees.

*This is an example of extreme axis deviation due to ventricular tachycardia.*

#### Example 5

- Lead I = isoelectric.
- Lead aVF = positive.
- This is the easiest axis you will ever have to calculate. It has to be at right angles to lead I and in the direction of aVF, which makes it exactly +90 degrees!

*This is referred to as a “vertical axis” and is seen in patients with emphysema who typically have a vertically orientated heart.*

#### Causes of Axis Deviation

**Right Axis Deviation**

- Right ventricular hypertrophy
- Acute right ventricular strain, e.g. due to pulmonary embolism
- Lateral STEMI
- Chronic lung disease, e.g. COPD
- Hyperkalaemia
- Sodium-channel blockade, e.g. TCA poisoning
- Wolff-Parkinson-White syndrome
- Dextrocardia
- Ventricular ectopy
- Secundum ASD – rSR’ pattern
- Normal paediatric ECG
- Left posterior fascicular block – diagnosis of exclusion
- Vertically orientated heart – tall, thin patient

**Left Axis Deviation**

- Left ventricular hypertrophy
- Left bundle branch block
- Inferior MI
- Ventricular pacing /ectopy
- Wolff-Parkinson-White Syndrome
- Primum ASD – rSR’ pattern
- Left anterior fascicular block – diagnosis of exclusion
- Horizontally orientated heart – short, squat patient

**Extreme Axis Deviation**

- Ventricular rhythms – e.g.VT, AIVR, ventricular ectopy
- Hyperkalaemia
- Severe right ventricular hypertrophy

#### Further Reading

For a deeper understanding of axis determination, including a detailed explanation of the hexaxial reference system, check out this excellent series of articles from EMS 12-lead.

- Chung DC, Nelson HM. ECG – A Pictorial Primer [internet]. Accessed 20/10/2014
- Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
- Marriott’s Practical Electrocardiography 12e
- EMS 12 Lead Axis Determination Part I
- EMS 12 Lead Axis Determination Part II
- EMS 12 Lead Axis Determination Part III
- EMS 12 Lead Axis Determination Part IV
- EMS 12 Lead Axis Determination Part V
- EMS 12 Lead Axis Determination Part VI
- EMS 12 Lead The 360 degree Heart